CRC32 C or C++ implementation

Use the Boost C++ libraries. There is a CRC included there and the license is good.

The crc code in zlib (http://zlib.net/) is among the fastest there is, and has a very liberal open source license.

And you should not use adler-32 except for special applications where speed is more important than error detection performance.

The SNIPPETS C Source Code Archive has a CRC32 implementation that is freely usable:

/* Copyright (C) 1986 Gary S. Brown.  You may use this program, or
   code or tables extracted from it, as desired without restriction.*/

(Unfortunately, c.snippets.org seems to have died. Fortunately, the Wayback Machine has it archived.)

In order to be able to compile the code, you'll need to add typedefs for BYTE as an unsigned 8-bit integer and DWORD as an unsigned 32-bit integer, along with the header files crc.h & sniptype.h.

The only critical item in the header is this macro (which could just as easily go in CRC_32.c itself:

#define UPDC32(octet, crc) (crc_32_tab[((crc) ^ (octet)) & 0xff] ^ ((crc) >> 8))

I am the author of the source code at the specified link. While the intention of the source code license is not clear (it will be later today), the code is in fact open and free for use in your free or commercial applications with no strings attached.