One More Slot Position objects (OMSPs) allow placement of objects in locations not normally permitted by the game. Uses include creating the appearance of clutter on top of other objects and positioning Sims for photography.
I believe that Whole New Dimension: OMSPs Revisited and Improved! and Full Range Shiftable OMSPs - Closer than we've ever been! by SilentLucidity at MTS should take care of all of your OMSP needs, in conjunction with the various Maxis objects with extra slots on the Fixed Maxis Objects page. If not, here are some other options:
Also get SilentLucidity's Get Where You're Going -- Walk Around & Through Blocks, which allows Sims to walk around or through objects that would normally block their path.
Another great OMSP related download is julsfels' OMSP Controller, which lets you hide/unhide all the OMSPs at once. Compatible with most of the OMSPs above.
On the other hand, if you have too many slots and your Sims insist on putting pizza boxes, mail, homework, coffee cups, etc. where they don't belong, get julsfels' Slot Blocker!
If you need extra seating in/on other objects, get Honeywell's Cop-A-Squat: Chair OMSPs and the loveseat OMSP included in Improved Cop-A-Feel: Loveseat OMSP For Beds.