Mark's Signing Bonus has been around as long as I can remember. It was the first site I visited when I put up my first site in May of 2001. The most consistent site you'll ever see and great information as well. This is the banner he used to use twenty years ago, still my favorite.
SportsCollectors.net or SCN for short has also been around as long as I can remember. Jay runs a great site with the largest TTM sports database on the net. It has grown into a juggernaut.
SportsCardForum or SCF for short is another site that has been around for a long time now. I have been a member there since 2003 and in the time since they've built a large TTM database. Check it out.
DN Autographs.com is a site that seems to have survived the purge of autograph sites. Dan and Neil have run their site since 2004. This is their old banner, check them out. They also have a twitter @DNAutographs.
Ross's Sports Autographs is another site that has been around a long, long time. Ross also posts his successes on his Facebook page
More Collector Links
Here are more collector sites in alphabetical order that are worth a look. If you'd like to exhange links, just drop me an email at firstname.lastname@example.org