The next is Mountain Lion:
Upgrading to Mountain Lion
To upgrade to Mountain Lion you must have Snow Leopard 10.6.8 or Lion installed. Purchase and download Mountain Lion from the App Store. Sign in using your Apple ID. Mountain Lion is $19.99 plus tax. The file is quite large, over 4 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.
Macs that can be upgraded to OS X Mountain Lion
1. iMac (Mid 2007 or newer)
2. MacBook (Late 2008 Aluminum, or Early 2009 or newer)
3. MacBook Pro (Mid/Late 2007 or newer)
4. MacBook Air (Late 2008 or newer)
5. Mac mini (Early 2009 or newer)
6. Mac Pro (Early 2008 or newer)
7. Xserve (Early 2009)
Are my applications compatible?
It depends upon how you look at it. Lion came after Snow Leopard so is "next" in terms of order. However if your computer is capable of doing it (which I somewhat doubt with a computer running 10.4.11 as the OP's profile suggests) then there is the option of going straight from Snow Leopard to Mountain Lion (10.8), with ML being "next" if there are no other limitations.