Greasemonkey

Greasemonkey, also known as grees@ is yet another usefull Firefox extension.
Here's a description from official Greasemonkey site:

Greasemonkey is a Firefox extension which lets you to add bits of DHTML ("user scripts") to any web page to change its behavior. In much the same way that user CSS lets you take control of a web page's style, user scripts let you easily control any aspect of a web page's design or interaction.

You can install Greasemonkey from this page!

What we are interested here is, how can it improve your Ogaming.
Most common uses (scripts) are:

• Directly send recycler, spy a planet or moon from galaxy
• Removes commander ad, and other ads
• Removes Commander info from left menu
• Show colored alliances in galaxy panel, variable size of debris field icon, adds arrival time to flights overview...

You can download Ogame scripts for Greasemonkey and install them just few clicks. No need to pay for Commander anymore.
If you're a bit advanced user, you can make a custom script yourself and add even more functionality. Diveintogreasemonkey.org can help you with the first steps.

I am unsure whether Greasemonkey is legal or not, you should contact GO's in your universe and ask. But even if it's not legal, there is no way you can be tracked if you use it.

Foxgame

Foxgame is a free Firefox extension, and surprisingly, it's legal.

Features:

• Automatic universe selector in login page.
• Reduced galaxy view
• Extra delete options and spy report options: You can control messages from the top or the bottom.
• Fleet arrival and return time: In fleet destiny selection page you will see the time when you fleet will arrive and come back to your planet.
• Fleet retreat time: In fleet page you will see the time when your fleet will arrive if you order to come back.
• Multilanguage support
• Almost all finishing hours: Research and buildings finishing hours, hangar's qeue finishing time, you can see date/time in overview if you wish and even in phalanx.
• Highlight ally and private messages: Ally and private messages now have a diferent background to see them easily.
• Highlight big debris fields: Debris fields bigger than you fix min will have different background in galaxy view.
• Autochoose mision type: You can define mission priorities so FoxGame will select the mission according to that.
• Online DBs integration: Send reports to Milos (only ogame.com.es), EspRep and GalaxieTool directly within OGame (solar systems, stats and spy reports supported).
• In mines and power plants info screens (the ones you see when you click in the name) you will see the diferences between your actual level and the rest.
• Autoselect fleet destination: Select coord text anywhere go to fleet send screen and Voila! the destination is already selected. There is no need to take down numbers anymore.
• Private message signatures.

I find it incredibly usefull, as it will save you a bunch of time. You can install Foxgame and see for yourself.