iStat Menus lets you monitor your system right from the menubar. Included are 8 menu extras that let you monitor every aspect of your system.

CPU
Realtime CPU graphs and a list of the top 5 CPU resource hogs. CPU usage can be tracked by individual cores or with all cores combined, to save menubar space.
Network

A realtime graph to keep on top of what’s being sent and received for all network connections.
Date & Time

A highly configurable date, time and calendar for your menubar, including fuzzy clock and moon phase. A world clock with sunrise, sunset, moonrise and moonset times for over 20,000 cities. It’s one of the most powerful replacements for Apple’s date and time menu available.
Disk Usage

See used or free space for multiple disks in your menubar. More detail for all your disks is only a click away.
Disk Activity

Detailed disk I/O in your menubar, displayed as a graph, a variety of different read and write indicators, or both.
Sensors

Realtime listings of the sensors in your Mac, including temperatures, hard drive temperatures (where supported), fans, voltages, current and power. Fan speeds can be controlled, with different rules when on battery power, if you’d like.
Battery

Detailed info on your battery’s current state and a highly configurable menu item that can change if you’re draining, charging, or completely charged.
Memory

Memory stats for your menubar, shown as a pie chart, graph, percentage, bar or any combination of those things. The memory dropdown menu shows a list of the top 5 memory hogs, as well as other useful info.
Complete control

Each menu extra comes with many different display modes, customizable colours, font sizes and widths.

Languages: English. Partial localization for French, German, Italian, Japanese, Spanish and Dutch.

Requirements: Mac OS X 10.5, 10.6 or newer

Version 3.19:
Final release for Mac OS X 10.5 and PPC. Future releases will require an Intel based Mac running Mac OS X 10.6 or newer.
Fixed bug that could cause GPU stats to stop updating.
Recovery partition in Mac OS X 10.7 is now hidden in the Disks extra.
Improved sensor support for Mid 2011 iMacs.

Download links (4% Recovery):