There are lots of reasons why you'd want to access your PC remotely, and luckily there are also plenty of ways to do so. Crossloop is one of the simplest (it's also free), but for those willing to venture further, you've got lots of options.
Excuse us while we get technical for a second. This list is divided into four main sections: VNC (Virtual Network Computing), NX, remote desktop and cross-protocol. If those terms mean nothing to you, you might want to skip to the "other" section for the most straightforward applications. That said, let's begin.
VNC
Ajax VNC - Clientless VNC solution in the web's favorite programming language.
Apple Remote Desktop - Apple's jumped on the remote desktop bandwagon with this great VNC server built right into OS X.
Chicken of the VNC - One of the most popular VNC clients for OS X under the GPL.
DirectVNC - UNIX client using the framebuffer device via DirectFB.
FVNC - Great VNC client written in Flash. That's right, Flash.
MetaVNC - This unique client/server package merges the host and remote desktops together into one Windows or Linux desktop.
.NET VNC Viewer - C# viewer that works with Windows and Windows Mobile/CE devices.
PowerVNC - Java-based client with support for SSH and SFTP.
RealVNC - Server/client from the original VNC developers.
sVNC - The perfect tool for offering remote assistance to Mac users, since there's barely any configuring required.
SymVNC - Symbian phone client for connecting to your PC.
TightVNC - Popular VNC server/client with nice Java applet.
TurboVNC - TightVNC spinoff with great support for 3D apps.
UltraVNC - Wildly popular client/server app with file transfer, video drivers, chat, and much more.
Vine Server - OS X server with clipboard sharing, port changing, and a universal binary.
VncViewer for 8086 - Got an old IBM running FreeDOS or DOS? Put it to work with this VNC client.
VNCViewer for PocketPC - Client for, you guessed it, PocketPC users.
x11vnc - Unlike most other VNC servers for UNIX and Linux, this server lets you share your current desktop session (instead of a separate session).
NX
FreeNX - Open source server using the libraries from NoMachine.
NoMachine NX Client - Free client to connect to NX machines.
NoMachine NX Free Edition - NX server for UNIX/Linux.
NoMachine NX Enterprise Desktop - Server that includes everything that the free edition has, but with unlimited users (unlike the free version's two).
NoMachine NX Small Business Server - NX server targeting small businesses.
NoMachine NX Enterprise Server - NX Server with unlimited users and unlimited connections.
NoMachine NX Advanced Server Subscription - Everything that the Enterprise server has, plus multi-node capabilities.
Remote Desktop Protocol
CoRD - Remote desktop client for Mac OS X users.
grdestkop - Gnome wrapper for the rdesktop application.
krdesktop - rdesktop frontend written in KDE.
properJavaRDP - Cross-platform Java client based on rdesktop.
rdesktop - Extremely powerful command-line client for UNIX/Linux.
Remote Desktop - Microsoft's official server, for XP Pro and Vista.
xrdp - Serves an X window session to incoming Windows clients.
Cross-Protocol
Connection Manager - VNC, RDP, and Telnet manager for UNIX/Linux distros.
KDE Desktop Sharing - The official KDE VNC server package.
KDE Remote Desktop Connection - The official KDE client for RDP and VNC.
Kurd - Great rdesktop/vncviewer frontend for KDE.
mRemote - Manage VNC and RDP connections on Windows with this handy app.
RDPMan - A little utility to manage RDP, VNC, and Telnet connections for Windows.
Other
CrossLoop - Easy-to-use remote access utility with file sharing and a multilingual interface.
GoToMyPC - Extremely popular remote access program for ~$25/month.
LogMeIn - Remote access app with a free and a pro ($12.95/month) version with file transfer, remote printing, and more.
Unyte - Nice remote desktop server with a Java applet viewer and IM integration.
See also:
50 Great Widgets For Your Blog
15 Ways To Create Website Screenshots
50+ Google Reader Extensions and Scripts For Firefox
WEB OS RUNDOWN: 45+ Web Operating Systems
AMAZON TOOLBOX: 40+ Amazon Tools
30+ Language Tools For Firefox
SKYPE TOOLBOX: 50+ Enhancements for Skype
MOBILE PRODUCTIVITY TOOLBOX: 45+ Mobile Productivity Resources
MORE THAN A BROWSER: 19 Alternative Ways to Use Firefox
FORUMS GUIDE: 70+ Resources for Creating a Forum
VOIP TOOLBOX: 30+ VoIP Services
P2P TOOLBOX: 30+ Peer to Peer Services
DIGG TOOLBOX: 50+ Digg Tools and Resources
WORDPRESS GOD:300+ Tools for Running Your WordPress Blog
GREEN LIVING TOOLBOX: 80+ Green Sites
Travel Toolbox: 75+ Online Travel Resources
ITUNES TOOLBOX: 30+ Tools For iTunes
COLLEGE TOOLBOX: 60+ Tools For Back To School
ONLINE PRESENTATIONS: 30+ Presentation & Slideshow Services
GMAIL TOOLBOX: 60+ Tools For Gmail
ONLINE BUSINESS TOOLBOX: 230+ Tools for Running a Business Online