As Ambient says, bot GarageBand and Logic have good drum sequencers built in, but if you are not using a Mac and you also want an application that works stand alone, then specifically Toontrack's EZ Drummer 2 has got to be one of the best in terms of sounds, ease of use and price. I use it virtually everyday and I'm still finding cool things to do with it.
http://toontrack.com
The loops that come with the app are arranged in a 'finder' so that you can either tap in a groove that you are looking for and it will list hundreds that match, or you can select by a playing style (say half-time rock) and it will offer up a whole pile of grooves that might suit a chorus, or an intro. Fills and breaks are also included.
One function I really like is that sometimes I find a cool groove but the kick drum doesn't match my bass line. That's ok as you can remove just one kit drum from a groove and replace it leaving all the other hit-hat and snare detail in place.
You can create songs structures in the time bar and when the whole thing is ready you can export your work. Or, you can do all of this from within your DAW of choice because the app also runs as a plug in. You can trigger it via MIDI using a keyboard or an electric drum kit.