General antonyms:
* Often
* Frequently
* Regularly
* Usually
* Always
* Continuously
* Constantly
More specific antonyms:
* Rarely (meaning "seldom" can also be an antonym of "rarely")
* Occasionally
* Sometimes
* Frequently
The best antonym will depend on the specific situation.
For example:
* "He seldom visits his parents" could be replaced with "He often visits his parents."
* "She seldom misses a day of work" could be replaced with "She rarely misses a day of work."