I've been researching this subject extensively for a cart I want to produce for a friend as an xmas gift. So here's what I've seen:
This site and ultimate-console.fr seem to have the most useful information. Though most people seem to have derived their knowledge from the sparse ROMLAB site.
slpcat's Blog has lots of good information as well, though not in much detail. He also has a few related videos on YouTube.
Rev Pon's Blog is another great source of information.
Nintendo Age also has a well documented cartmod in the issue of their zine from 10/2009, which can be downloaded for free once you register.
SNES Central has lots of technical information about cartridges.
Digit Press's forum also had some info.
EPROM's
Cartmods can be accomplished with various EPROM's including:
27C801 - DIP32 Package ( 8-bit )
27C322 - DIP32 Package ( 16-bit, uncommon )
29F032 - TSOP40 Package ( Flash )
When searching for these, you'll come across various versions from different manufacturers. If product code contains one of the codes above, then it based on the same design and should be interchangeable.
You'll find most tutorials making use of the 27C801, because it works for most games and is the most easy to accomplish.
It's also important to note that the 27C801 and 27C322 aren't being manufactured anymore (from what I've read), so their prices are rising. You can always erase used ones, but new ones are becoming more rare.
EPROM Programmer
Acceptable and cheaper EPROM programmers included the Willem and the GQ-3X/GQ-4X.
Most people don't recommend building one. Though if you want to save money, the EZoFlash is a Willem compliant option, some people have done this, but if you're new to electronics work you'll save time and anguish buy buying a cheap one.
You should also consider a UV EPROM eraser if you use the 27C801 or 27C322 if you need to rewrite them.
People recommend using the SNES power supply with the Willem instead of USB power for reliability reasons.
Process
The easiest way to get your game ready is to use another game, usually referred to as a donor cartridge, with a PCB that supports the features you need. Generally you remove, what is called, the MaskROM and replace it with an EPROM burned with your new game on it. You need to pay attention to if the game you want to write is HiROM or LoROM, and choose a donor cartridge accordingly, SNES Central has lots of info on this.
For the 27C801 & 27C322, this generally involves making minor alterations to the PCB and adding additional wires to your new EPROM because the pins on the newer EPROM's don't match those of the MaskROM's. Some games require multiple EPROM's and another IC to tell the cart how to use them together. The disadvantage to this is when you need many EPROM's, it can be difficult to make them all fit nicely in the cartridge.
When you're working with the 29F032, you'll generally need to use TSOP to DIP36 adapter board. This circumvents the need to alter the original cartridge PCB resulting ins a cleaner looking mod. It's also important to note that the size of the 29F032 is 4x that of the other EPROM's and therefore is better suited to larger games. The disadvantage is that you'll need to buy or made the additional TSOP to DIP36 adapter. This involves soldering the 29F032 to the adapter board, and this chip's leads are very tiny. Most people recommend tutorial videos on YouTube to help with technique. Here are a couple good ones. The other challenge is that you'll need another adapter to program the 29F032, because it isn't in a DIP form factor, your EPROM programmer cannot use it without one. These can be very expensive. More than some of the cheaper programmers themselves! It is also possible to build a programmer that will let you program the chip from the DIP36 adapter board. A schematic for this is on the ROMLAB site.
buiICnow.com sells the TSOP to DIP36 adapter boards. Many people have designed them, most seem to be working from the design on ROMLAB. I have seen people report success for both of the boards ( Type I and Type II ) listed on the site. As a side note it's important to remember to bridge the resistor contacts on the Type I with either 10 ohm resistors or just solder. Most people claim you don't need the resistors, and that a solder bridge is sufficient.
Once your EPROM is burned it's a matter of soldering it to the cartridge PCB and testing the cartridge. Many people have also made tools, such as cartridges with sockets to allow easy swapping of EPROM's to help with testing.
Edit 12/16/11: BUYICNOW has added a "Type III", which just appears to be a Type II? The numbering is reversed in the PCB image, but other than that all of the traces appear to match identically. Odd. Well I have just ordered 4, so I tell you if I notice anything strange.
I'll happily further update this with more info as I gather it. The challenge is that this information is really scattered across many forums and languages. Remember to use Google Translate!
