They have become far less valuable (almost none now) for passing PR, but for internal linking its still a little valuable, especially if there arent very many.  But for the most part footers are no longer valuable - Google cracked down on that a while ago.

Best practice for all three cases is to never use the file extensions.  You should never link to the file extension names, and make sure in your htaccess file that you dont use the file extensions for any reason moving forward.  Why?

1. Lets say you decide to re-do your site and it goes from PHP to another language like ASP or something.  You would have to redirect your entire site with file extensions and would shoot yourself in the foot with SEO, traffic and anything else.  By not using file extensions, you give yourself the flexibility down the road and you can maintain a constant url structure.

2. Indexing may or may not use the file extensions depending on your htaccess/server settings.  You would then essentially be running into duplicate content pages and issues, and thereby negatively affecting your site.  Plus, it will dilute your individual page authority.

As a side note, just be consistent with your internal linking.  Whether you use relative links or not - some discussion can be had around that.  But pick a route and go with it, just as long as you dont use the file extensions

I think you may be getting xml sitemaps confused with sitemap pages.  Your xml sitemap should live at /sitemap.xml as Alan pointed out.   The seomoz and other sites that have a /sitemap page is for different purposes.  Its not your xml file, its a "topical guide" to your website and all the major sections of your site.

Remember that you can also create a xml sitemap index if you need to have different sitemaps (video, news, content) that houses all the different xml sitemaps underneath it.

Domain authority can have a big influence on page authority.  Its not a direct correlation, because page authority depends on a lot of different inputs on a page level (internal links, external links, information architecture, perceived value).  But just because a site have good DA doesn't mean that all internal pages get a nice PA.

Depending on the value of those three backlinks, three could be all you need to boost a PA.

1. File names- always use the hyphen.

2. Alt Text - use spaces and normal word structure.  Ryan is correct on the technical intent behind the alt tag, but it can also be a good SEO tool, and you should treat those as exact phrase keywords.  So nobody is searching for "city-bikes", they are searching for "city bikes".  The alt tag should be what the user will put into google.

I second this opinion.  folders are the best way to go here.

I second all the comments below.  That should give you enough professional consensus in this arena to head in the right direction.   Redirect with teh keywords, and this will have the best long-term results.

As for Google parsing keywords out of urls or anything else...  You will need to remember that for the most part google ignores special characters and parameters in both content and urls.  It doesnt matter if the url is in a link or the actual url bar up top, google will treat it the same.  Google knows its a url string, and applies its "url parsing logic" to it.  So it only makes sense that a full url link would be parsed, just the same way your website url is parsed because it includes the keywords.   Its the same logic.

Technically this just creates a link farm, of which the search engines can easily draw the connections.  They know that you, linking to them, and them linking to these other sites are all connected.  Give them 6 months and they will have all the lines drawn between your dots

Not the best long term strategy.... but I also know that this still works in the real world (so I've heard.....:)  Short term you can get some valuable link juice there, and will probably see some benefits.  But know that you could get smacked down one day.

I am starting a Google doc today, and will start populating directory links with information.  Anyone that wants to help put the new list together, shoot me a personal message from my profile here and i will share the doc with you!

I will probably start a new Q&A post specifically for this so we can collaborate and have a dedicated section- so keep your eyes out and join the discussion.

Along those same lines, does anyone else know of any good services that will provide physical business addresses in certain locations?  I know of some virtual office providers, but only found ones that do major cities, not smaller ones.

1. File names- always use the hyphen.

2. Alt Text - use spaces and normal word structure.  Ryan is correct on the technical intent behind the alt tag, but it can also be a good SEO tool, and you should treat those as exact phrase keywords.  So nobody is searching for "city-bikes", they are searching for "city bikes".  The alt tag should be what the user will put into google.

They have become far less valuable (almost none now) for passing PR, but for internal linking its still a little valuable, especially if there arent very many.  But for the most part footers are no longer valuable - Google cracked down on that a while ago.

I second all the comments below.  That should give you enough professional consensus in this arena to head in the right direction.   Redirect with teh keywords, and this will have the best long-term results.

As for Google parsing keywords out of urls or anything else...  You will need to remember that for the most part google ignores special characters and parameters in both content and urls.  It doesnt matter if the url is in a link or the actual url bar up top, google will treat it the same.  Google knows its a url string, and applies its "url parsing logic" to it.  So it only makes sense that a full url link would be parsed, just the same way your website url is parsed because it includes the keywords.   Its the same logic.

I am starting a Google doc today, and will start populating directory links with information.  Anyone that wants to help put the new list together, shoot me a personal message from my profile here and i will share the doc with you!

I will probably start a new Q&A post specifically for this so we can collaborate and have a dedicated section- so keep your eyes out and join the discussion.

Technically this just creates a link farm, of which the search engines can easily draw the connections.  They know that you, linking to them, and them linking to these other sites are all connected.  Give them 6 months and they will have all the lines drawn between your dots

Not the best long term strategy.... but I also know that this still works in the real world (so I've heard.....:)  Short term you can get some valuable link juice there, and will probably see some benefits.  But know that you could get smacked down one day.

Domain authority can have a big influence on page authority.  Its not a direct correlation, because page authority depends on a lot of different inputs on a page level (internal links, external links, information architecture, perceived value).  But just because a site have good DA doesn't mean that all internal pages get a nice PA.

Depending on the value of those three backlinks, three could be all you need to boost a PA.

I think you may be getting xml sitemaps confused with sitemap pages.  Your xml sitemap should live at /sitemap.xml as Alan pointed out.   The seomoz and other sites that have a /sitemap page is for different purposes.  Its not your xml file, its a "topical guide" to your website and all the major sections of your site.

Remember that you can also create a xml sitemap index if you need to have different sitemaps (video, news, content) that houses all the different xml sitemaps underneath it.

I second this opinion.  folders are the best way to go here.