Durable Water Repellent (DWR)

Durable Water Repellent (DWR) finishes are hydrophobic coatings applied to fabrics to make them water-resistant by causing water to bead up and roll off fabrics, rather than soaking into them. They are often used in conjunction with certain waterproof/breathable fabrics to prevent the outer layer of a laminated waterproof/breathable from becoming saturated with water. Maintaining the DWR is critical to maintaining the breathable nature of waterproof/breathable outerwear.
Despite the name, durable water repellent finishes tend to wear off and may need to be re-applied from time to time. The application method will probably make a big difference in the DWR you choose to use. Some, like the Nikwax boot finish, are thick pastes that are applied by hand while the vast majority can be poured or sprayed on. Many are applied by adding to a regular wash cycle in a washing machine.
Common brands of DWR finishes include:
Nikwax (uses patented wax-based elastomer - non fluoropolymer)
Granger's (fluoropolymer base)
Tectron (fluoropolymer base)
ReviveX (no information available, most likely industry standard; fluoropolymer base)
Deluge (fluoropolymer base)
Often the best way to choose a good DWR is to visit an outdoor gear store such as REI or Eastern Mountian Sports and talk to a sales rep who has tried a number of different finishes.
WARNING: While fluoropolymers offer a high level of water repellency, as well as a high degree of oil and stain repellency, everyday users should be aware that fluoropolymer compounds can degrade into PFOA's. PFOA's are being reviewed by the E.P.A. (United States Environmental Protection Agency] to determine their potential for human carcinogenicity. One of the only exceptions to this is the above-mentioned Nikwax, which uses a patented, environmentally-safe wax elastomer.