How do I link to Google Maps with a particular longitude and latitude?

The best way is to use q parameter so that it displays the map with the point marked. eg.:

https://maps.google.com/?q=<lat>,<lng>

This schema has changed again (23rd October 2018). See Kushagr's answer for the latest.

This for a map with the marker (via aaronm's comment):

https://www.google.com/maps/?q=-15.623037,18.388672

For an older example (no marker on this one):

https://www.google.com/maps/preview/@-15.623037,18.388672,8z

The oldest format:

http://maps.google.com/maps?ll=-15.623037,18.388672&spn=65.61535,79.013672

As of today (2014/09/23), I've found that to get marker on exact place (not an approximation) you can use:

http://www.google.com/maps/place/49.46800006494457,17.11514008755796

Additionally, if you want to specify map center and zoom:

http://www.google.com/maps/place/49.46800006494457,17.11514008755796/@49.46800006494457,17.11514008755796,17z

If you want to use satellite map type, then append /data=!3m1!1e3

http://www.google.com/maps/place/49.46800006494457,17.11514008755796/@49.46800006494457,17.11514008755796,17z/data=!3m1!1e3

And If you want terrain view of the map, then append /data=!3m1!4b1

https://www.google.com/maps/place/49.46800006494457,17.11514008755796/@49.46800006494457,17.11514008755796,17z/data=!3m1!4b1