If you use gmail you can read your personal email at work without being snooped on. Just always visit the secure site, at https://mail.google.com/ This prevents them from snooping on you if they read email on the network. Also if they block webmail most blocking firewalls and proxies will not block ssl secure sites, like the secure version of gmail. On the other hand if the individual computers have the spying software rather than at the network level they can still read whats being typed and/or see screenshots. Unfortunately, there's no way of knowing which method is used without violating company policies.
If you use hotmail, yahoo, or another webmail provider you can still use gmail in this secure way by setting up mail fetching and "sending email as another email address" under settings. Hotmail recently allowed POP3 access for all users including non-paying ones. Gmail recently has started using a service that connects to yahoo and a few other webmail providers to fetch your email to gmail from the other non-gmail address.
The reason for doing the above is that while yahoo and hotmail have secure logins they don't let you read your email securely, like gmail does.