Example talking to the Desk.com API from Salesforce (Apex).

DeskSample.java

public class DeskSample
{
    private static String OAUTH_KEY = 'youroauthkey';
    private static String OAUTH_SECRET = 'youroauthsecret';
    private static String ACCESS_TOKEN = 'youraccesstoken';
    private static String ACCESS_TOKEN_SECRET = 'youraccesstokensecret';
    
    public static String DESK_SITENAME = 'yoursite';
    
    public static Void doRequest()
    {
        HttpRequest req = new HttpRequest();
        req.setMethod('GET');
        req.setEndpoint('https://' + DeskSample.DESK_SITENAME + '.desk.com/api/v1/cases.json?count=10');
        // req.setBody(''); // you can use that in combination with a POST request
        
        // we need to sign the request
        req = DeskSample.signRequest(req);
        
        Http client = new Http();
        try {
            // execute the web service call
            HttpResponse response = client.send(req);
            
            // debug messages
            System.debug(response.getBody());
            System.debug('STATUS: ' + response.getStatus());
            System.debug('STATUS CODE: ' + response.getStatusCode());
            
            // process the response
            
            // return the response
        } catch(System.CalloutException e) {
            System.debug('EXCEPTION THROWN: ' + e.getMessage());
        }
    }
    
    private static Map<String, String> getParams(String paramString)
    {
        Map<String, String> params = new Map<String, String>();
        
        if (paramString == null || paramString == '') return params;
        
        for(String s : paramString.split('&')) {
            String[] sl = s.split('=');
            if (sl.size() == 2) {
                params.put(sl[0], sl[1]);
            }
        }
        
        return params;
    }
    
    private static HttpRequest signRequest(HttpRequest req)
    {
        Map<String, String> params = new Map<String, String>{
            'oauth_consumer_key' => DeskSample.OAUTH_KEY,
            'oauth_nonce' => String.valueOf(Crypto.getRandomLong()),
            'oauth_signature_method' => 'HMAC-SHA1',
            'oauth_timestamp' => String.valueOf(DateTime.now().getTime()/1000),
            'oauth_token' => DeskSample.ACCESS_TOKEN,
            'oauth_version' => '1.0'
        };
        
        String[] host = req.getEndpoint().split('\\?');
        
        // parse get parameters
        if (host.size() == 2) {
            params.putAll(DeskSample.getParams(host[1]));
        }
        
        // parse body parameters
        if (req.getBody() != null && req.getBody() != '') {
            params.putAll(DeskSample.getParams(req.getBody()));
        }
        
        // create the base string
        String baseString = '';
        List<String> keyList = new List<String>(params.keySet());
        keyList.sort();
        for (String key : keyList) {
            baseString += key + '=' + params.get(key) + '&';
        }
        baseString = req.getMethod().toUpperCase() + '&' +
                     EncodingUtil.urlEncode(host[0], 'UTF-8') + '&' +
                     EncodingUtil.urlEncode(baseString.substringBeforeLast('&'), 'UTF-8');
        
        System.debug('BASE STRING: ' + baseString);
        
        // create the signature
        Blob sig = Crypto.generateMac('HmacSHA1', Blob.valueOf(baseString), Blob.valueOf(
                        DeskSample.OAUTH_SECRET + '&' + DeskSample.ACCESS_TOKEN_SECRET
                   ));
        String signature = EncodingUtil.urlEncode(EncodingUtil.base64encode(sig), 'UTF-8');
        
        // create the header
        String header = 'OAuth ';
        for (String key : params.keySet()) {
            header += key + '="' + params.get(key) + '", ';
        }
        header += 'oauth_signature="' + signature + '"';
        
        // sign the request
        System.debug('Authorization: ' + header);
        req.setHeader('Authorization', header);
        
        return req;
    }
}