Some every quarter hour. Every hour. Once a day. It's too many processes for cron to cope with. The every minute ones, for example, I want to stagger over the minute so they're not all firing right bang on the zero seconds mark.
So, you need 4 shell (or perl or whatever) scripts like: minute.sh 15min.sh hour.sh day.sh which sit in some directory somewhere. Each script then runs all the jobs to be run on that schedule and does so sequentially (or uses GNU parallel or similar if you want a farm). Then your cron config only needs 4 entries and cron can quite happily cope with that.
* * * * * /foo/minute.sh
*/15 * * * * /foo/15min.sh
0 * * * * /foo/hour.sh
0 0 * * * /foo/day.sh