One day I'll use an oxygen tank or maybe shake a carboy to aerate my wort, but for now I'm fermenting in a bucket and using a drill mounted aerator which seems to work well. They sell drill mounted aerators (a.k.a. wine degassers) for $20-30. Plastic paint mixers are cheaper but may not be food grade plastic, if you care about that.
I made one from one of those cheap white plastic mash paddles which I already owned and which I assume many homebrewers also already own. They're like $5-8 at the LHBS. You just need the mash paddle, a paddle drill bit (funnily has the same name), and some tape. I these three things around so it was free, though I did buy these things earlier.
The mash paddle has a flat end as does the paddle bit, so they stay together very easily.
I realize it is pretty ghetto, but it seems to work as well as any of the other drill mounted tools I've seen. I didn't take any pictures of it in action, but it got my wort very foamy when I tried it on my last batch. I used two packets of dry yeast and it was bubbling like crazy, about once every second or two, about 2 hours after pitching.
Paddle bit & flat end of mash paddle:
Tape the drill bit to the mash paddle and voila. I used duct tape, and it worked great. Remove when you're done and you can use the paddle again next time. If the tape leaves adhesive residue, WD-40 works well to clean it off.