Practically, I think Julie is right, but I have seen heavy devaluation of these footer links in the past year or two. They'll still count for something, but not a lot. The only warning I'd add is that I wouldn't create a situation where these are your ONLY links. You could risk looking like a link farm and even a potential penalty at that point. These easy links should be only one part of your link-building strategy.

I'd also highly encourage diversity. Mix up the anchor text, as long as it's relevant, and maybe even put the link different places. If you can get contextual links somehow (not footers or sidebars), that's a huge plus. The more you can mix it up, the better.

There's no "have to" in most of these situations, but it boils down to this - the more canonical your canonical URL actually is, the better chance you have of Google honoring it. In other words, if you set a canonical tag but then never use that in internal links or your XML sitemap, odds are pretty good that Google may ignore the tag in some cases. You're basically saying "Hey, this URL is canonical! No, this one is! No, this one!" - it's a mixed message, and they're going to try to interpret it algorithmically.

I definitely think pointing to yet another version in the XML sitemap is a problem. Ideally, it would be great to unify your URLs, but if that's not possible, getting the canonical version in the sitemap would be a big help (and introducing yet another variant isn't good, so you'd kill two birds with one stone). As Andy said, if you could create some kind of internal link to the canonical version, even if it's not the main link, that could also help. I only hesitate on that one, because you don't want to end up with a weird, artificial linking structure (just creating links to have links).

Please note, this isn't necessarily a disaster the way you have it. Google could honor the tags properly and generally rank your site correctly. In my experience, though, it's a recipe for long-term problems, and it's worth fixing.

I'm actually not sure I agree. From a theoretical, PageRank-passing perspective, sitewide links are better. From a penalty/risk perspective, 1000s of sitewide links can lead to a ton of links coming from very few unique domains, which can start to look suspicious. I actually think you might see less devaluation by limiting the footer links to a couple of strong pages on each client site.

Hooray! Usually, I just give my advice and then run away, so it's always nice to hear I was actually right about something Seriously, glad you got it sorted out.

A couple of options here. First off, though, there's really no rel="noindex" at the link level. You can "nofollow" a link, and that generally disrupts indexing, but it's not guaranteed. You're right that it can look like PR sculpting, although that's not a huge issue if your usage makes sense. In other words, if you're using rel=nofollow to keep the crawlers away from content with low search value, I generally think that's ok.

You could META noindex, nofollow the target pages, although then Google has to crawl those. The advantage is that I find the META Robots approach to be a bit more powerful.

The other option is to use parameter handling in Google Webmaster Tools (Bing has a similar function) to tell Google to ignore the "?filter" parameter. The purist in me doesn't love the engine-specific approach, but it's easier, you don't need to change the site itself, and it typically works fairly well.

I'm not a fan of doubling up, but only because it makes the results really hard to measure. If you implement both, you won't know which one worked, ultimately. I'm not sure it's actually harmful - it just can be hard to track.

If you're just trying to prevent future problems (and don't have any immediate issues), I'd probably pick one and give it a few weeks.