I've seen a lot of fanart at some tumblr sites. Because the titles to the websites are a little strongly worded I put this under spoiler tags. I'll take these down if there's a problem with posting them.
http://fuckyeahboostergold.tumblr.com/
http://fuckyeahjli.tumblr.com/There's a lot of stuff at the boostle livejournal but be warned that a lot of it has Ted/Booster as a pairing.
http://boostle.livejournal.com/tag/fanficThe first fics I read were LMs' at fanfiction .net, she writes the league not long after Booster joins. Her stories are funny and/or touching.
http://www.fanfiction.net/u/30037/LM Here's some summaries of some of her more Booster related fics:
There's
Road Rage where J'onn has to drive the whole league on a trip, it's funny and mentions a few things that happened to Booster then.
Cultural Significance has Ted teaching Tora and Booster how to hold chopsticks--it's funnier than you'd think.
Secret Indignities where Booster takes an upset Ted Kord out to lunch where they chat and try to avoid someone.
All That Glitters is a more serious take on Michael Jon Carters' early years that shows how certain things helped forge him into the man he becomes. It's not completed but worth checking out.
Justice League Ponyland I haven't read this in a long time and it took me a while to get pass my own misgivings. From what I remember it's VERY cracky and dives right into how completely insane it would be for Booster and Beetle to be transported into the My Little Pony world. And the ponies themselves act off the wall or in one case actually insane. Not what I expected at all but I remember cracking up at the humor. I don't think it's completed.