While we've all heard of the effects that can come from blowing your CL to epic proportions, I haven't heard a lot about what you can do with increased Spell Level (SL). I've found a dozen uses, ranging from broken to extremely broken.
The basic build I always start out with is a level 5 Illumian wizard. He takes the Improved Krau Sigil feat for +2 to SL on 2 spells, and the Enhanced Sigils feat to boost that to +3 (As far as I know, nothing in the wording of either feat prevents Enhanced Sigils from affecting an Improved Sigil). Then he takes Sanctum Spell (I guess this build gets flaws; really, the same concept works at higher levels, so the number of feats doesn't matter a lot), boosting his maximum SL to 7 (a +4 bonus on a 3rd level spell). Now, he can technically cast 7th level spells, which gives him a few options:
Storm Bolt: As long as he keeps a 3rd level spell, the possibility that its effective SL can become 7 allows him to use a 7d6 lightning attack at will. I picked Storm Bolt rather than other reserve feats because it is the only one that neither requires an attack roll nor allows a Reflex save.
Spell Versatility: This transmuter ACF lets the character learn a new spell of up to the highest level spell he can cast, giving him a free 7th level spell at level 5. It's always great to be ahead of the game, especially when it involves making Limited Wishes.
Extra Slot: You gain a spell slot at a level lower than the highest SL you can cast. This means that you get a 6th level slot to fill with anything at least a few levels in advance. Take it a second time, and you could have 9th level spells at level 9. Go even further than that, and persistent shapechange becomes a reality with minimal effort by level 15.
I'm sure that there are tons of other uses that I haven't thought up yet. The possibilities are endless.