Because nodes.dat is just a list of IPs (no copyrighted content), it is legal to share anywhere. However, you must be careful of malicious sites distributing fake files. Here are the trusted, active sources as of today.
At its simplest, nodes.dat is a text file containing a list of IP addresses and UDP ports of other eMule clients. Think of it as a phonebook for the eDonkey network. When you first install eMule, you don't know where anyone is. The nodes.dat file gives you the initial "contacts"—a handful of other computers that are currently online and active on the network.
Once eMule reads this file, it contacts those IP addresses. Those computers then introduce your client to their list of known friends. Within minutes, your client builds its own dynamic routing table, and the nodes.dat file becomes largely obsolete until the next time you start from scratch or lose connectivity.
The Golden Rule: nodes.dat is only useful as a starter key. If you haven't connected in months, you need a fresh nodes.dat. If you are connected, you never need to manually update it again—eMule will save a new one automatically upon exit.
In the eMule IRC-style console (bottom of the window), type:
bootstrap [IP address] [Port]
If you know one active node (e.g., from a friend), you can bypass the file entirely.