If you want just to get the value, you can use simple shell oneliner like this:
S=10; F=/sys/class/net/eth0/statistics/rx_bytes; X=`cat $F`; sleep $S; Y=`cat $F`; BPS="$(((Y-X)/S))"; echo $BPS
It will show you the average "received bytes per second" for period of 10 seconds (you can change period by changing S=10 parameter, and you can measure transmitted BPS instead of received BPS by using tx_bytes instead of rx_bytes). Don't forget to change eth0 to network device you want to monitor.
Of course, you are not limited to displaying the average rate (as mentioned in other answers, there are other tools that will show you much nicer output), but this solution is easily scriptable to do other things.
For example, the following shell script (split into multiple lines for readability) will execute offlineimap process only when 5-minute average transmit speed drops below 10kBPS (presumably, when some other bandwidth-consuming process finishes):
#!/bin/sh
S=300; F=/sys/class/net/eth0/statistics/tx_bytes
BPS=999999
while [ $BPS -gt 10000 ]
do
X=`cat $F`; sleep $S; Y=`cat $F`; BPS="$(((Y-X)/S))";
echo BPS is currently $BPS
done
offlineimap
Note that /sys/class/... is Linux specific (which is ok as submitter did choose linux tag), and needs non-archaic kernel. Shell code itself is /bin/sh compatible (so not only bash, but dash and other /bin/sh implementations will work) and /bin/sh is something that is really always installed.
