You mean like
http://api.stackoverflow.com/1.1/tags
which gives:
{
"total": 35690,
"page": 1,
"pagesize": 70,
"tags": [
{
"name": "c#",
"count": 576794,
"fulfills_required": false
},
{
"name": "java",
"count": 554383,
"fulfills_required": false
},
{
"name": "javascript",
"count": 523194,
"fulfills_required": false
},
...
Have a look at Stack Exchange API