GTunnel

I just tried GTunnel from Garden Networks

It's free, but I don't *think* it's open source. I wrote GN and asked about it.

It has the following features:

• Easy to use user interface, English-Chinese dual language support, automatic recognition of OS language
• Local HTTP/Socks5 proxy (UDP is not supported yet) with intelligent recognition of HTTP/Socks5 protocol
• Automatic configuration of both global (system-wide) and per-instance proxy setting in Internet Explorer
• Support multiple transport modes to provide high service availability and flexibility in various network environment. In the standard mode, GTunnel will connect to servers of our server farm directly. In Skype Mode, GTunnel will try to connect via the P2P (Peer To Peer) network of Skype. In Tor mode, GTunnel connects throuph Tor nodes and adds security to Tor users. In GTunnel Tor mode, even the Tor exit node owners do not see the original traffic.
• Automatic software upgrade
• 128 bit industry strength data encryption with highly randomized traffic pattern
• Runs on Linux through Wine.

NOTE:
It also works through Firefox and Opera.
It's not officially classed as a portable app, but rather it's a self-contained executable that doesn't require an installer, so I'm using it on my thumbdrive as a portable nonetheless. I also have a copy on my laptop.