ULS Log Reader from SPSDev: http://www.spsdev.com/UlsReader.aspx.
Realtime? ULSViewer wins hands down: http://www.harbar.net/archive/2010/10/06/ulsviewer-ndash-the-tool-that-no-sharepoint-practitioner-should-be.aspx
Update: ULS has been resumed officially with updates (Aug 2014) https://blogs.technet.microsoft.com/wbaer/2014/08/22/uls-viewing-like-a-boss-uls-viewer-is-now-available/
If you are looking to dive deep and investigate all facets of the issue, including the ULS, IIS, event logs and even performance logs, you might want to look into the SharePoint Diagnostics (SPDiag) tool that is part of SharePoint Administration Toolkit, a free tool from Microsoft.
This compiles all of the logs from all servers across the farm lets you get a bird's eye view of what was going on at the given time.
It does take a few steps to set up but really can help troubleshoot an issue.
Documentation can be found on TechNet.
Stefan Gordon's ULS Viewer
Also have a look at http://spcorrelationviewwp.codeplex.com/, which allows you to retrieve the information recorded in the ULS log tagged with a specific correlation ID without leaving your site since it's a webpart.
UlsViewer is a no brainer, you got to have it. But for debugging realtime on several web front end servers I still prefer SPTraceView by Hristo Pavlov.
This unique tool lets you watch ULS errors as they occur from multiple WFE. You install it on all WFE and one is chosen as receiver. This way you can monitor behaviour even though you don't know what WFE your users are having errors with.
It only supports MOSS currently.
Try SharePoint Log Reader on codeplex. The ability to filter on category and correlation id is great. It's a WPF application.
The Merge-SPLogFile cmdlet is invaluable when working in a multiserver farm environment. It collects the log files from all servers into a single local file. It supports basic filtering to reduce the size of the log file that is collected.
I want to link to my personal favourite: the MIT licensed SharePoint Log Viewer. With it one can see all logs from the entire farm live, filter by all fields, and much more.
The SharePoint ULS Viewer by Dan Winter, Microsoft.
Here is a list of SharePoint ULS log viewers out there along with comparison of features and the verdict on which one is the best:
SharePoint LogViewer - View ULS logs the easy and efficient way http://sharepointlogviewer.codeplex.com/
Not being satisfied with any of the SharePoint log viewers out there, I created my own
You can also check our tool SysKit Insights. We took a different approach to solve this problem, we do gather logs similar to other tools mentioned, but we also provide you with a full-blown search engine to query these logs. The built-in alerting system allows you to subscribe to important alerts that can be sent as exceptions are logged into ULS logs. We offer Express edition of Insights for free.