My searches led me to consider three classes of printers - standard point-of-sale (POS) printers, consumer-style address label printers (ALP), and the thermal printer mechanisms that are common to both. To summarize, old used POSs can be a great option for cheap and easy text printing ($30+). New POSs are too expensive ($200+). ALPs give you more flexibility with images, a neat sticker form-factor, and come at reasonable prices ($90+) - but they require more work to interface to. Mechanisms are a pain, but can be a cheap and flexible solution if you have the time and knowledge to build a driver circuit and write control software (I won't go into them more here, check out Nigel's thermal Printer notes). What follows is a summary of important issues...

There are two types - printers that print images and those that print text. Older POSs are text printers that generally have a parallel port, and can be used really easily - by just dumping ASCII characters at them (I have other notes about this). Those that print images generally have USB and/or serial and use some special protocol to communicate between the PC (and thus come with a printer driver). Essentially you dump a bitmap to them over serial. POSs and ALPs almost all now use USB/serial. All the ALPs I found print images. All the new and expensive POSs do images as well.

Vendors supply printer drivers with their POS printers, and some even offer expensive solutions for Windows and OSX. If you're using a regular computer, that's great, but I needed to control them from embedded products, so it was important that they have good documentation. Some POS vendors wanted me to sign an NDA to get their specification. The ALP vendors often have their specification up on their website publicly available (like Seiko).

Most important! New POS printers cost about $250 - crazy. I found a handful of older POS models that were available at surplus stores and on eBay for around $40 each. This is reasonable, but keep in mind that I didn't find any that did images at that price, they were all of the text type that just do ASCII characters.

A great ALP is the Seiko SLP100, which can be found for less than $100 new. Their specification is online. They even provide sample code! Grab their developer SDK. I'll probably release a Java-base OSX printing app for it, so then it would be usable with a USB->Serial adaptor. However, address labels are clearly a lot different than a nice scroll of receipt printer paper - most importantly, they are stickers!