Workflows can be scheduled to run every 30 minutes, though it would take a bit of work to get the email to contain all of the saved search results. It doesn't look like more granularity than that is offered, though.
Scheduled or Map/Reduce scripts are much more granular and allow you to specify repeating every 15 min, 30 min, or every 1, 2, 4, 6, 8, or 12 hours. Of course, scripts take a bit more overhead to set up and write.
Saved searches only allow scheduling to send once daily, as you mention.
The simplest solution would be to create multiple copies of the saved search you want to be sent; one for each time of day you want it sent. This requires a lot of maintenance of course: if you change one of the searches, you need to change them all to be identical.
The best solution is the scripted one, though it is the most costly.