The type of pano stitcher you want depends on a lot of different factors, such as the type of panoramas you're stitching and how much ease of use you want, and the hardware you're using.
If you just want to experiment with a handful of photos with free or already-owned software, and you want the most ease-of-use, the Windows-only application, Microsoft ICE, is probably the easiest to try. Similarly, if you purchased a Canon camera that came with a disk in the box, then Canon's Photostitch might be worth a go. There's also Autotstich, and Photoshop's PhotoMerge capability.
However, these "easy to use" stitchers typically don't give you a lot of control over how the images are stitched and combined, and while they'll work terrifically for most panos, for a few trickier types of panoramas, there may be stitching errors or ghost/clones that are more difficult to remove. And for gigapixel panos with a large array of shots, or mixing shots from different cameras/lenses, or sets with parallax error, or 360x180 sets taken with fisheye lenses, these stitchers may begin to struggle and might not accomplish the task you need them to do, and you might need to move to a more specialized stitcher.
Quite of a few of these are based upon Helmut Dersch's panorama tools. Hugin and PTGui are probably the most popular of these. There is also Max Lyon's PTAssembler (which he wrote to stitch the first gigapixel pano). These stitchers have a steeper learning curve, but more tools for correcting panos. And Hugin, in particular, has become a front-end gui for a number of different open source line command graphics manipulation tools, and can see a lot of "off-label" usage, such as focus stacking, lens correction, defishing, remapping, alignment, etc. that isn't strictly about stitching panos.
Autopano Pro is based upon Autostitch instead of Panorama Tools, but also has a great reputation as a pro-quality stitcher with great ease of use. And Gigapan offers their own Gigapan Stitch to go with their camera mounts to make super-high resolution gigapixel panos.