Long story short, If you're installing a client-side mod that doesn't add any new blocks, items, or entities, you'll be fine, If you get a mod that does add items, blocks, or entities, you will start to encounter some problems with LAN but you won't see as many on an acctual server. If you're talking about putting mods on the server, all players will have to have that mod client-side to connect. Yet, if they're mods that add items and blocks into the game, like BuildCraft or RedPower, as long as your minecraft versions are the same you should not encounter any errors when trying to connect, yet, if you attempt to get one of the items in the mod out of a Creative inventory, or another inventory, you will be kicked from the server.
If you're talking about an actual server, then any mods that do not add anything into the game, like I said above, ShaderMod, TooManyItems, and others, you will not encounter any problems. But if it's a mod that adds new items, like RedPower, or BuildCraft, then you will start to encounter problems with LAN. If you're talking about LAN, then any mods that do not physically add anything into the game, such as TooManyItems, a minimap mod, or the shader mod, you should not encounter any problems. It depends on what type of mod you are talking about, and whether you're are talking about LAN or actual servers.
