For OpenTTD you can use the original Transport Tycoon Deluxe data files (you need to own a Transport Tycoon Deluxe CD). There are also the free alternatives: OpenGFX (graphics), OpenSFX (sound) and OpenMSX (music). These can be installed automatically by the Windows and OS/2 installers. Please refer to the readme for more information.
For all binaries officially released by us we publish the MD5, SHA1, and SHA256 checksums. You can use these checksums to check wheter the file you downloaded has been modified. All three checksums should match the file you downloaded; if this is not the case it means that either the file didn't come from us or that it got broken during transport. Either way it might possibly contain dangerous modifications and the file should therefore not be trusted!
Latest release in openttd-useful is 4.2, released on 2011-07-27 18:30 UTC.
[ 22.0 MiB ][ Checksums ]md5sum : 0aa66ecfaec70ca056a99f0351763c6b
sha1sum : 42d8ffde7f1391254d7246c9d278114c48566652
[ 11.2 MiB ][ Checksums ]md5sum : f9bc1ee99a78026cb2b99b83089f361c
sha1sum : be9569d305abba2cb8a9d434c1944fcc6f15459e