Below is a list of the programs I use to make tilesets. If you're thinking of making a tileset, I highly recommend downloading these and giving them a try.
I've used them all for years now and they do a great job.

Program: Comment:
Paint Shop Pro The most versatile image editing program available. It's like this program was made to make tilesets. The amount of features are endless, give the demo a try now!
TeraLogic Texture Maker A great program to make textures, and in the case of tilesets, textured backgrounds. Comes with enough features for the professional user, but also has a very easy way to make textures for someone less experienced.
Palette Suite 3.01 The greatest palette editing program I've come across, and it's made by Toxic Bunny from our very own Jazz 2 community! This is a must download if you plan on making tilesets.
TilesetPal 1.1 Another very useful program, and this one was created Monolith. It comes in handy for editing palettes and most notably reducing the colors of images. I personally like it because often times it does a much better job of reducing colors than Paint Shop Pro and other programs. -BlurredD
Default Jazz 2 Palette This is the default Jazz Jackrabbit 2 palette in .pal format (used with Palette Suite and Paint Shop Pro). This palette is a must for every tileset, and a great starting point for someone who has no experience with palette editing.