Having shell accounts on one or more remote Unix systems can enhance portability by allowing you to compile and test code (as well as view manpages) for systems to which you don't have local access. They are also useful for testing your systems in that they allow you to access your local network (mail, http, etc.) from "outside". If you need access to a particular type of Unix system, this is the place to look.
If you need access to a type of Unix system that isn't listed below, or if you need full shell access to a particular variant of Unix, please let us know, and we will do our best to accomodate you at very affordable rates.
UnixPorting maintains accounts on most of the following servers, so you can be assured they are useful and reliably available.
unixssh.com: FreeBSD; provides access to GCC compilers, Perl, Python, etc.
devio.us: OpenBSD; provides access to GCC compilers.
SDF [telnet here] offers NetBSD 1.6.1 running on DEC Alpha 21246DPs. Established in 1987, SDF is a non-profit 501(c)7 membership sponsored organisation. Basic shell access is free, and members have access to development tools and other features.
Nyx, "the world's oldest free public-access Internet Service Provider, provides Unix shell accounts, a full, uncensored newsfeed, email, web space, and even Unix programming and development tools for non-commercial use."
Polarhome: Linux, OpenVMS, Solaris, AIX, QNX, IRIX, HP-UX, Tru64, FreeBSD, OpenBSD, NetBSD and OPENSTEP.
HP offers the Developer & Solution Partner Program , which offers, among other things, remote access to a variety of HP and Compaq servers.
IBM runs two programs, PartnerWorld and Linux Community Development, that offer access to AIX and other IBM OSes, and Linux on the S/390, respectively. The Linux S/390 program is free; unfortunately IBM has placed some constraints, including set times at which you are able to register.