Inspired byhttp://thomasstreet.net/blog/spreadsheet.html. Features:
 
 

  
  
Under 30 lines of vanilla JavaScript
  
  
Libraries used:none
  
  
Excel-like Syntax (formulas start with "=")
  
  
Support for arbitrary expressions (=A1+B2*C3)
  
  
Circular reference prevention
  
  
Automatic localStorage persistence
 
 

li {
  list-style: none;
}
li:before {
  content: "✓ ";
}


input {
  border: none;
  width: 80px;
  font-size: 14px;
  padding: 2px;
}


input:hover {
  background-color: #eee;
}


input:focus {
  background-color: #ccf;
}


input:not(:focus) {
  text-align: right;
}


table {
  border-collapse: collapse; 
}


td {
  border: 1px solid #999;
  padding: 0;
}


tr:first-child td,td:first-child {
  background-color: #ccc;
  padding: 1px 3px;
  font-weight: bold;
  text-align: center;
}


footer {
  font-size: 80%;
}

for (var i=0; i<6; i++) {
    var row = document.querySelector("table").insertRow(-1);
    for (var j=0; j<6; j++) {
        var letter = String.fromCharCode("A".charCodeAt(0)+j-1);
        row.insertCell(-1).innerHTML = i&&j ? "<input id='"+ letter+i +"'/>" : i||letter;
    }
}

var DATA={},INPUTS=[].slice.call(document.querySelectorAll("input"));
INPUTS.forEach(function(elm) {
    elm.onfocus = function(e) {
        e.target.value = localStorage[e.target.id] || "";
    };
    elm.onblur = function(e) {
        localStorage[e.target.id] = e.target.value;
        computeAll();
    };
    var getter = function() {
        var value = localStorage[elm.id] || "";
        if (value.charAt(0) == "=") {
            with (DATA) return eval(value.substring(1));
        } else { return isNaN(parseFloat(value)) ? value : parseFloat(value); }
    };
    Object.defineProperty(DATA,elm.id,{get:getter});
    Object.defineProperty(DATA,elm.id.toLowerCase(),{get:getter});
});
(window.computeAll = function() {
    INPUTS.forEach(function(elm) { try { elm.value = DATA[elm.id]; } catch(e) {} });
})();