There are so many. A friend of mine got a $99 wireless Lexmark AIO unit "free" at the Apple store when he purchased an iMac the other day. I like Brother. I use a MFC-7440n. It is a network monochrome laser all in one fax/scan/copy/printer. The only down side is that Brother toner isn't cheap. The drivers are flawless and ship with OS X.
I don't like HP. I don't like their toner prices nor do I like their software. I don't have much of an opinion about Epson or Canon. I have avoided Lexmark over the years but now that they are being offered at the Apple store perhaps they are worth another look. My son has one of those Kodak AIO units with the "low price" toner. All I know is he isn't constantly pestering me about ink.
Make sure you pick a printer that mentions OS X and or Linux on the outside of the box. No digging through the manual to find out if it supports OS X. If they make you look that hard to find it, the driver might not be very good.
I suggest a HP J4500. Set up was quite possibly the easiest thing I've ever done and it works perfectly. It has all the features (fax, scan, print, etc.) and it was very inexpensive.
i just picked up an Epson WorkForce 840 for $120 that works pretty well. The only thing I don't like is I have to perform scans over USB vs. the network interface.
My Ubuntu laptop prints fine to it over the network as well.
I would check this list for SL and Lion compatibility: