Here's what I got
Starting with the must have list:
You must have a life jacket (personal floatation device or PFD) for everyone on board. Anyone under 12
must wear the PDF at all times while on board. Anyone over 12 is not required to wear it, but you must have one "readily available" for everyone. Everyone, regardless of age, who rides in a towable, water skis or wake boards must wear a PFD. I got the nicer, more comfortable Type III vests for our family and we picked up some inexpensive Type II vests at Wal-Mart (since guests rarely wear them on board and we can loan our nicer vests for the tube riders.
You also need a Type IV floatation device (ring buoy also known as a life preserver). This needs to be USCG approved, which means it's much more expensive than those cheap pool rings you'll see. If you get stopped by harbor patrol, you could be cited for not having the proper required equipment.
You need an oar. I recommend getting one that collapses. I found one pretty inexpensively on Amazon that collapses down to about 2 feet, so it stores in the floor storage easily.
You need at least a 2 lb fire extinguisher rated A & B. You can get a rechargeable 5 lb ABC extinguisher at CostCo very inexpensively. I think it was no more than $20.
If your boat has a working horn, that qualifies as your audible signaling device. Whether you decide to get another is up to you. You can get whistles that work even when wet for a couple of dollars, or a compressed air blaster.
If you'll be out on coastal waters (think ocean), you'll need one of several different kinds of visual signaling devices (like flares, smoke signals, etc.) This is not necessary for inland waters (i.e., lakes, etc.) I'm not sure where the Great Lakes fall on this requirement.
Not required, but a good idea:
2 or three fenders to protect your boat when docking. You can get 5"x20" Taylor Made fenders for $12 each online
Rope for docking and fenders. I just picked up 3/8" x 100' nylon rope at Home Depot and cut the lengths I needed. Much cheaper than buying pre-cut segments. It's marine-rated.
Buy a small plastic bin with cover (Wal-Mart or Target) about 12"-15" long by 6"-8" wide. Inside, put these items:
Wrench (like a ratchet wrench with a socket) to change spark plugs
3 NGK spark plugs
Some Mr. Clean Magic Erasers
A couple of kitchen-sized (13 gallon) trash bags. I like the ones with the built-in handles/ties.
Waterproof sunscreen
Some extra feminine hygiene products
A couple extra gallon-sized Zip-Loc bags
iPod adapter cable for the stereo (unless you aren't planning on keeping the stereo for any length of time). This allows you to control the iPod with the stereo (and the remote, including seeing the song if your remote has a display). About $15 online.
Put your registration, insurance papers, and boat manuals in a gallon-sized Zip-Loc
A first-aid kit. Sam's Club has a nice kit in a plastic bin with over 300 pieces, everything you'll need short of needing to perform surgery

, and it's about $20.
A "water-skier down" flag. Not many states require it, but it's a good idea, and they are not expensive. I picked on up with a suction cup mount (so you don't have to hold it for $5 online).
A larger plastic bin that you can fit in the floor storage (size depends on your storage area and door size, but my boat's door is only about 12" wide and I easily got an 18"x 24" bin. I put towels in there for everyone.
A tube/towable for the kids
A "Booster Ball" for the towable or, if you opt not to get the booster ball, get a tow rope that has Bungee® in it
Pump for towable (make sure it's a high-volume type)
Stair grip tape (it's like 2" wide sandpaper with glue on the back. You can get this at Lowe's or Home Depot in a 15' roll). I use it to put on the trailer on any part where I might want to step.
A boat cover from Poppy Company.
http://www.poppycompany.com They're not the cheapest, but these covers are awesome. High quality, fit perfectly, easy to install, don't flap in the wind, let you fill up the boat with gas without taking of the cover, etc. Tell them you're a member here and ask for the Forum discount. I think it's 10% with free shipping.
Good to get sometime in the near future:
Extra trailer tire and mount for trailer (eBay)
Tire iron (I like the four-way, cross-shaped ones for best leverage)
Quick-change trailer jack (like this one, which stores easily):
http://www.wholesalemarine.com/p/S-M-1840046/
8' anchor (fluke style is most popular)
VHF radio (I recommend a portable, with dual channel capability--they're pretty cheap)
Hydro-Turf kit. Get 15% with the code "HYDRO15". Hopefully your boat didn't come with the carpet kit, because you're better off without the carpet and putting the money toward Hydro-Turf. You can buy sheets yourself cheaper, but you'll have to do all the cutting. I decided just to get the professionally pre-cut kit for ease and saving my time.
Some extra 5-gallon gas cans for longer boat outings so you don't have to pull the boat and go to a gas station or pay marina gas prices. Also pick up some silicon tube to attach to the can spouts because the new style cans don't have extendable spouts any more. Plan about 18" per can.
A cooler that fits in your storage bin/area so that you can bring it from the house/car right to the boat and drop it in. The Sea-Doo ones are pricey, but they fit perfectly.
Meguiar's M4516 Polish and M5616 Wax