Splurge #1: A large camera bag. I have used a Lowepro bag for quite some time and it never fit all the gear I need for a shoot. So I kept some of the gear in the bag and simply held the other stuff or kept it in the manufacturer's lens bag. I never bought a bigger bag until I dropped a $2,500 lens because I was juggling too much gear. That experienced helped me to realize that you NEED a protective camera bag or you are risking the breakage of your gear. In terms of bags, I recommend the Lowepro Fastpack 250 for hobbyists who don't have large pro lenses, or the ThinkTank Streetwalker Harddrive for more serious photographers who have a few larger lenses, or the Clik Elite Pro for photographers with a lot of gear who like the backpack style bags.
Splurge #2: A QUALITY circular polarizer. That $20 polarizer you bought at the camera store might look okay in your shots until you try a decent one. In my experience, the difference between a cheap polarizer and a decent one can be quite stunning. If you're in the market for a good polarizer, you need to look on your lens to see what size you need. Then, pick a brand. I really like the Nikon polarizer and have heard good things about the Singh Ray polarizers (I haven't personally used the Singh Ray polarizer, but I use their other products and can vouch for the company).
Splurge #3: A decent ballhead. It seems that most beginning photographers heed the advice to buy a solid tripod; unfortunately, when they buy a decent tripod, they often buy the legs that includes a cheap 3-way pan head on it. Those cheap heads simply won't cut it. They suffer from creep, are difficult to precisely adjust, are overly bulky, and won't last more than a year in most cases. If you want to step up to a good ballhead, the cheapest one that I can recommend is the Vanguard BH-100. If you are ready to buy the best ballhead that money can buy (and you'll need a lot of money…), then check out the Really Right Stuff ballheads.
Splurge #4: A true wide-angle lens. I don't think I've ever taught a landscape photography class without at least one student asking why photographers spend $1,000 or more on a 10mm lens when the kit lens already goes down to 18mm (for crop sensor cameras). There is a HUGE difference between a 10mm lens and an 18mm lens. For more on wide-angle lenses, check out this previous post.
Splurge #5: Stabilized lenses. I liken this argument to the manual vs. automatic debate for cars. Sure, a manual transmission car might save you $300 on the purchase price, but automatic so much more smooth and simple! Modern image stabilization can recover up to FOUR STOPS! Yes, the stabilization is always worth it.
Splurge #6: Name brand batteries. I tried to save a buck once on a cheap camera battery. It was half the price of the Canon battery, but it didn't last nearly as long and leaked power. Many of the cheap batteries are not certified, so they can actually be dangerous to you and can hurt the camera by heating up too much. It's just not worth saving $20 on a battery to risk a camera that cost hundreds or thousands of dollars.
Splurge #7: A REAL underwater housing for your DSLR. I am not an underwater photographer, but since I lived near the ocean, I have seen several photographers ruin their DSLRs with a cheap $100 underwater housing. If even a hair from your head falls on the Zip-loc part of the bag, you can kiss your DSLR goodbye. If you want to dabble in underwater photography, buy a Panasonic underwater point-and-shoot or get ready to spend $1,000 on a real underwater housing.
Splurge #8: A camera strap. The strap that came with your camera might be free, but it also saws through your neck during a long shoot. Check out this review of the Black Rapid strap, and then buy yourself a Black Rapid R-strap on Amazon for Father's Day whether you're a father or not.