A lot of people have probably heard of VisualBoyAdvance, a Game Boy/Super Game Boy/Game Boy Color/Game Boy Advance emulator. It's one of the most popular emulators for the Game Boy series, but it also has a lot of bugs. There are also many versions that improve on it in different ways, which are all made by different people, so they are all separate.
VisualBoyAdvance-M aims to combine all these improvements into one emulator, and to make a better emulator overall. It's currently under development right now, with only a few people working on it, so it'll take a while for it to be complete. With only a few people, though, a lot has already been done, and in the direction that it's going, I'm pretty impressed with it. For one, the Full Screen feature works a lot better than previous versions -- it's less buggy, and can be done simply by hitting Escape to switch in and out of it. There are also more video filter options, and it's easier to use overall. A few bugs have been fixed, but some others are still around (such as the making AVI videos, which I nitpicked about here: http://vba-m.ngemu.com/forum/avi-record ... t-187.html), and there are even a few new ones (savestates are kinda glitchy), but they're working on fixing them.
Oh, and the sound is based off 1.8.0, which is much better than the sound in 1.7.2 (which is the version that almost everybody uses, sadly). Playing DKL3 sounded horrible in 1.7.2; it sounds much better here. Same with all other GB games, but DKL3 is one of the most affected ones.
Here's the site for this emulator: http://vba-m.ngemu.com/
You can try out the emulator and see how you like it. You need 7-zip (http://www.7-zip.org) to decompress the emulator if you download it.