// see: http://community.salesforce.com/t5/Apex-Code-Development/Building-a-reusable-JSON-component/m-p/172428
    // and: http://joe-ferraro.com/2009/04/extjs-vf-json/
    
    public static String getJsonFromObject(String objectName, List<String> columns, String filterString)
     {
     	if(filterString == null)
     		filterString = '';
  
	     	String jsonData = '[ ';
			try {
				
				//System.debug('getJsonifiedObjectRecords for:' + objectName);
				  
				String queryString = 'Select id,';
				  
				for(String c: columns) 
					queryString += c + ',';
				
				queryString = queryString.substring(0,queryString.length() - 1);
				queryString += ' from ' + objectName + ' ' + filterString;
				queryString += ' limit 10000';
				
				//System.debug('QUERYSTRING:' + objectName);
				List<sObject> sos = Database.query(queryString);
				//Map<String, Schema.SObjectField> columns = sos.getSObjectType().getDescribe().fields.getMap();
									  
				//jsonData += '[';    
		        for (sObject so : sos) {  
		        		jsonData += ' { ';
						  for(String column : columns)
				          {   
				          			System.debug('jsonData:' + jsonData);
				                     try 
				                     {
				                     	String cellval = '';
				                     	List<String> cParts = column.split('[.]');  
								    	if(cParts.size() == 2)
								    	{
								    		String outerObjectName = cParts.get(0);
								    		//if(outerObjectName.endsWith('__r'))
								    		//	outerObjectName = outerObjectName.substring(0,outerObjectName.length() - 1) + 'c';
											cellval = (String) so.getSObject(cParts.get(0)).get(cParts.get(1));
								    	}
				                     	else
				                     	{
					                     	cellVal = String.valueOf( so.get(column));
				                     	}
				                        jsonData += '"'+column+'" : "'+cellVal+'",'; 
				                        
				                          
				                     }   
				                     catch(Exception ex) 
				                     {
										System.debug(ex);
				                     }
						 }
			 			 jsonData = jsonData.substring(0, jsonData.length() - 1);
	                	 jsonData = jsonData + '},';  
						 
		        }
		
				jsonData = jsonData.subString(0,jsonData.length() - 1);
				
	        }
			catch(Exception e)
			{	
				System.debug('Bad query in Jsonify Call ' + e);			
			}

		jsonData += ']';
     	
      	System.debug('jsonData:' + jsonData);

        return jsonData;
   }

@fractastical Jsonify SObject 

Other