Similar meaning, but slightly less boisterous:
* Chuckle
* Giggle
* Snicker
* Laugh
* Cackle
More forceful, possibly with a hint of malice:
* Guffaw
* Roar with laughter
* Bellow
* Snort
Other options:
* Rumble
* Wheeze
* Snicker
The best synonym for "chortle" will depend on the context and the specific sound you want to evoke.